3 things keep a gasoline engine from running - lack of fuel to air ratio, lack of spark, and lack of compression. All three can be affected when a mower sits. Fuel can go bad, even if fuel is drained, rust can form in a tank from moisture in the air, and new fuel can knock it loose. Ethanol attracts water from the air. Fuel can get gummy and plug things in the carb. Batteries can go bad, mice can chew wiring. Valves can stick. You need to investigate all three of those areas to figure out which one is the cause. And it can be multiple causes, not just one. Fuel can go bad, a valve can stick, and mice can chew the wiring. No one here can tell you over the Internet which one it is. Basic engine troubleshooting will be needed for each one, but the tests are really simple.